1. Field of the Invention
The present invention relates to a method and an apparatus that coat the powder on a substrate placed inside the vacuum chamber continuously and uniformly by spraying the powder entrained on the carrier air which is sucked in from the outside at atmospheric pressure without any devices supplying specific gas.
2. Background Art
The conventional coatings spraying the powder on a substrate are briefly described as follows;
1. Cold Spray
Cold spray coats 1-50 μm metal particles on a substrate by spraying them with the pressurized gas that temperature of it is a few hundred° C. and pressures range from 10 to 40 bars. The spray velocity of the gas in the cold spray is more than 300 m/s. The particles are coated as being deformed plastically by the kinetic energy caused by the velocity of gas and heat of gas when they collide with a substrate. The thickness of the coating layer in the cold spray ranges from a few millimeters to a few centimeters. But the weakness of cold spray is that either fine particles or ones having low specific weight are not coated because their velocity noticeably decreases by the shock wave or the aerodynamic drag occurring shortly after the gas impinges upon the substrate.
(1) U.S. Pat. No. 5,302,414 (“Gas-dynamic spraying method for applying a coating”; PCT/SU90/00126) relates to a spray coating technique that uses 3 different ways to feed powder by the pressurized gas. The first method shown in [FIG. 1] of the patent shows that a compressed gas is transported to a pressure pipe and to a hopper containing powder and then the powder mixed with gas is transported to a nozzle by spinning a cylinder drum which is able to adjust pressure properly to prevent powder from flowing backward. The second shown in [FIG. 4] of the patent shows that a compressed gas is sent to a feeder containing powder directly and pushes away powder into a nozzle. The third shown in [FIG. 5] of the patent shows that a compressed gas is transferred to a heating unit and a feeder separately, and the heated gas and the powder are mixed in a mixing chamber which is connected to a carrier gas pipe and a powder feeding pipe, and the powder is sent to a nozzle smoothly without backflow.
(2) U.S. Pat. No. 6,139,913 (“Kinetic spray coating method and apparatus”) is about a spray coating technique. As shown in [FIG. 2] of the patent, pressurized gas is transported to a premix chamber and the powder is sent to the mixing chamber under higher pressure than the carrier gas.
(3) U.S. Pat. No. 6,569,245 (“Method and apparatus for applying a powder coating”) relates to an apparatus that powder under atmospheric pressure and the highly compressed carrier gas are transported to the nozzle unit and mixed in the ejection cap, and then they are sprayed on a substrate. As shown in [FIG. 1] of the patent, there is a problem that velocity of the powder can decrease after the powder hits the profile-shaping plate inside the nozzle unit and thus it cannot reach necessary coating velocity at the ejection cap. Another problem is that the powder can be coated on the profile-shaping plate, not the target substrate.
(4) U.S. Pat. No. 4,815,414 (“Powder spray apparatus”) shows a coating technique that powder under atmospheric pressure is transported to a nozzle unit and coated on a substrate. As shown in [FIG. 1] of the patent, powder in a feeder and compressed air in the compressed air supplier are sent to a nozzle unit and then ejected through a nozzle for coating. In this apparatus, pressure of the nozzle unit must be lower than atmospheric pressure in order for powder under atmospheric pressure to be transported to the nozzle unit. It may give rise to powder backflow. It, therefore, becomes more difficult to transfer powder to the nozzle unit when pressure of the compressed air increases to make spraying velocity of powder faster.
(5) Korea Pat. No. 10-0770173 (“Cold spray apparatus”), Korea Pat. No. 10-0575139 (“Cold spray apparatus with gas cooling system”), and Korea Pat. No. 10-0515608 (“Cold spray apparatus with powder preheating system”; PCT/KR04/03395) relate to a method transporting and coating powder by using a nitrogen gas, a helium gas, and an air as the conventional cold spray.
(6) Korea Pat. No. 10-0691161 (“Method fabricating field emitter electrode”) relates to the method fabricating the field emitter electrode with carbon nanotube powder by application of cold spray. But it also failed to overcome the problems shown from cold spray.
(7) U.S. Pat. No. 6,759,085 (“Method and apparatus for low pressure cold spraying; PCT/US03/18758; WO 03/106051) relates to the method recycling expensive pressurized inert gas which is usually used in Cold spray apparatus. The cold spray nozzle placed in the vacuum tank sprays powder transferred by the pressurized gas. And after the gas is sprayed, it is filtered through the vacuum pump and the carrier gas is compressed again by the gas compressor and recycled.
As described above, the weaknesses of the conventional cold spray are 1) needing a compressed gas supplier for the compressed air or the expensive inert gases such as nitrogen and helium to transport powder, 2) having difficulty in keeping on feeding a fixed amount of powder because it must be fed by force (injection), 3) generating a loud noise because it sprays in the air under atmospheric pressure, 4) being not able to spray on some substrates which cannot stand heat from the high temperature gas or heated powder, and 5) having difficulty in coating the powders which have a low specific gravity, much agglomeration between particles, or sub-micrometer size particles because velocity of powder noticeably decreases by the shockwave.
2. Aerosol Deposition
Aerosol Deposition developed and improved from Gas Deposition made it possible to fabricate a variety of thin layers. It is a key concept of aerosol deposition that carrier gas flows into the aerosol chamber containing powder and the powder and the gas are mixed and formed into the aerosol which is transported to the deposition chamber by the difference of pressure between the aerosol chamber and the deposition chamber and then the powder is deposited on a substrate by being blown through a nozzle in the vacuum deposition chamber.
(1) Korea Pat. No. 10-0767395 (“Composite structured material” PCT/JP2000/007076), Korea Pat. No. 10-0695046 (“Method for forming ultrafine particle brittle material at low temperature and ultrafine particle brittle material for use therein” PCT/JP2003/006640), and Korea Pat. No. 10-0724070 (“Composite structured material and method for preparation thereof and apparatus for preparation thereof” PCT/JP2000/007076) relate to the technique that applies the aerosol deposition method to coating. They show the spraying method that the pressurized gas generated by the gas compression units such as a gas compressor, a nitrogen tank, and a helium tank is transported into the aerosol chamber and mixed with the powder to make aerosol. And aerosol is transferred to the vacuum chamber through the carrier pipe and sprayed through a nozzle. Therefore, pressure of the pressurized carrier gas must be higher than one of the vacuum chamber in order for the powder to be transported to the subsonic nozzle.
However, it is very difficult to keep a fixed amount of aerosol coming out of the aerosol chamber uniformly and continuously.
(2) Korea Pat. No. 10-531165 (“Method and apparatus for carbon fiber fixed on a substrate”; U.S. Pat. No. 7,306,503 (“Method and apparatus of fixing carbon fibers on a substrate using an aerosol deposition process”)) has disclosed that in addition to a basic principle of aerosol deposition, the aerosol chamber can directly generate carbon nanotubes inside it and the carbon nanotubes generated in the aerosol chamber are mixed with gas and transported to a deposition chamber to be deposited on a substrate by a nozzle. The technique was applied to form a thin layer which was expected to be as good as a thin metal layer. But it was not successful since it was not possible to make a thin layer with uniformity and low sheet resistance by the aerosol deposition technique. The shape of a carbon nanotube particle is very different from one of a metal particle. It is a tube type and has a peculiar aspect ratio of diameter (dozens of nanometers) to length (dozens of micrometers), 500˜1.000 fold, which is completely different from a metal particle. And the carbon nanotube powder shows an agglomerate state by a Van der Waals force and an entangled state by a high molecule chain. These properties of the carbon nanotubes are the cause of the powder feeding problem and have been the obstacle to manufacturing commercialized large size products which absolutely need uniform coating.
Korea Pat. No. 10-0499613 (“Manufacture method for electron-emitting device, electron source, light-emitting apparatus, and image-forming apparatus”) and Korea Pat. No. 10-0490112 (“Method of producing fiber, and methods of producing electron-emitting device, electron source, and image display device each using the fiber”) disclose the method manufacturing devices such as an electron-emitting device using the above mentioned method.
(3) Korea Pat. No. 10-0846148 (“Deposition method using powder material and device thereby”) relates to the technique applying aerosol deposition which coats a thin layer at room temperature by keeping the adequate pressure enough to accelerate the velocity of particles inside the deposition chamber. But there is a problem coating continuously and uniformly because when adjusting the pressure to get necessary pressure, velocity of powder changes which means that there is difficulty getting a uniform coating layer. The aerosol chamber has a filter or a windmill to disperse the entangled powder, but it could produce the opposite effect on dispersion and the filter could make the flow rate of carrier gas worse. It results in unsteady feeding of powder and being not able to form a uniform coating layer.
This technique tried to make the impact energy of the powder stronger as increasing the velocity of the particles of the powder being sprayed through the nozzle by opening and closing the carrier gas pipe intermittently. However, the maximum velocity of the particle at the subsonic orifice-type nozzle exit is Mach 1 at best. As pressure of the nozzle inlet increases as opening and closing the carrier gas pipe intermittently, the mass flow rate through the nozzle increases, but the velocity of the particles of the gas and the powder at the nozzle outlet does not. So Aerosol Deposition, likewise Cold Spray, also needs the compressed gas supplier for carrying the powder and has a problem getting a uniform coating layer because a concentration of powders cannot be controlled.
In conclusion, both Cold Spray and Aerosol Deposition have the following weaknesses which need to be improved. 1) They require the pressurized inert gas or the compressed air higher than atmospheric pressure to carry the powder. 2) They have a problem feeding and spraying a fixed amount of powder continuously. 3) They have difficulty in coating the powders which have a low specific gravity, highly agglomerated particles, or sub-micrometer size particles.